1. Field of the Invention
This invention primarily relates to a vacuum booster device for boosting the master cylinder of an automotive hydraulic brake or clutch or the like with a vacuum force. More particularly, the present invention relates to a vacuum booster device of the type having the construction in which a booster piston is axially slidably accommodated in a booster shell for dividing, in cooperation with a diaphragm, the interior of the booster shell into a front side first working chamber in communication with a vacuum supply source and a rear side second working chamber in communication selectively with the first working chamber or the atmosphere via a control valve means operatively connected to an input member, said diaphragm being superposed on the rear surface of a disc portion of the booster piston and having an inner circumferential bead connected to the booster piston and an outer circumferential bead connected to the circumferential wall of the booster shell; both front and rear walls of the booster shell are connected to each other by tie rods extending through the disc portion of the booster piston and the diaphragm; and seal means are provided for through-holes in the booster piston.
2. Description of the Prior Art
The booster device of the above-described kind is used in such a manner that the rear end portions of the tie rods are fixed to a vehicle body serving as the support wall and the master cylinder is fitted to the front end portions of the tie rods. During operation, the device is able to transmit the forward thrust load, applied from the booster piston to the master cylinder, to the support wall via the tie rods thereby to prevent the booster shell from bearing the load. Accordingly, the booster device of this type has the advantage that the booster shell need not have such high rigidity as to withstand the load and can be formed from thin steel sheet, synthetic resin or the like material to reduce its weight.
However, generally in the known devices, when the booster piston is manually operated for actuating the master cylinder while the vacuum supply source is in an inoperative state and hence, when no vacuum pressure is accumulated in the first working chamber, the air inside the first working chamber is compressed by the advancing booster piston to produce discharge resistance against the vacuum supply source, hindering the advance movement of the booster piston so that its manual operation is likely to become unsteady.